1. Field of the Invention
The present invention relates generally to a Hybrid Automatic Retransmission reQuest (HARQ) method and apparatus in a mobile communication system. More particularly, the present invention relates to a method and apparatus for efficiently performing an Automatic Retransmission reQuest (ARQ) operation in an upper layer using a transmission status of an HARQ apparatus in a mobile communication system.
2. Description of the Related Art
The Third Generation Partnership Project (3GPP) is a standard group in charge of standardization of the Universal Mobile Telecommunication Service (UMTS) system. Long Term Evolution (LTE) is currently under discussion as the standard for the next generation mobile communication system. The LTE which is aimed at deployment in 2010 is a technology capable of implementing high speed packet-based communication at about 100 Mbps. Several schemes are also currently under discussion. For example, the schemes include one scheme for reducing the number of nodes located in a communication path by simplifying a configuration of the network, and another scheme for approximating radio protocols as close to a radio channel as possible. A configuration of the LTE is expected to change from the existing 4-node configuration to a 2-node or 3-node configuration. For example, a configuration of the LTE, as illustrated in FIG. 1, will be simplified to the 2-node configuration of an Evolved Node B (ENB) and an Evolved Gateway GPRS Serving Node (EGGSN).
FIG. 1 is a block diagram illustrating an exemplary configuration of a general LTE mobile communication system as an evolved mobile communication system. As illustrated, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified to a 2-node configuration of Evolved Node Bs (ENBs) 120, 122, 124, 126 and 128, and Evolved Gateway GPRS Serving Node (EGGSNs) 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 via the E-RANs 110 and 112.
In FIG. 1, the ENBs 120, 122, 124, 126 and 128, which are network entities corresponding to the legacy Node Bs, are connected to the UE 101 via radio channels. Compared with the legacy Node Bs, the ENBs 120, 122, 124, 126 and 128 perform complex functions. In LTE, all user traffic including real-time traffic such as Voice over IP (VoIP) service traffics will be serviced through a shared channel. Therefore, there is no need for an apparatus for collecting status information of UEs 101 and performing scheduling thereon. For example, the ENBs take charge of the scheduling.
Similar to High Speed Downlink Packet Access (HSDPA) or Enhanced Dedicated Channel (EDCH) for providing high-speed packet service, LTE also performs a Hybrid Automatic Retransmission Request (HARQ) for packet retransmission between the ENBs 120, 122, 124, 126 and 128 and the UE 101. HARQ refers to a technique for soft-combining previously received data with retransmitted data without discarding the previously received data if a reception error occurs at a receiver. This technique increases a reception success rate. The high-speed packet communication system such as HSDPA and EDCH uses the HARQ in order to increase transmission efficiency. LTE also uses the HARQ between the UE and the ENB.
In the evolved high-speed packet communication system, such as LTE, it is not possible to satisfy various Quality-of-Service (QoS) requirements only with the HARQ. Therefore, outer Automatic Retransmission Request (ARQ) can be performed in an upper layer, and the outer ARQ is also performed between the UE and the ENB. LTE will use Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in the 20-MHz bandwidth in order to implement a data rate of a maximum of 100 Mbps. In addition, an Adaptive Modulation & Coding (AMC) scheme for determining a modulation scheme and a channel coding rate according to a channel status of the UE will be applied to LTE.
FIG. 2 illustrates an exemplary radio protocol structure and packet structure in a general LTE system. The radio protocol structure of FIG. 2 is applied to the ENB and the UE.
Referring to FIG. 2, a radio protocol of the LTE includes ARQ entities 210 for performing retransmission in upper layers, a Medium Access Control (MAC) layer 215 where an HARQ operation is performed, and a physical (PHY) layer 217. One ARQ entity 210 can be provided for each individual service, and satisfies required QoS through an outer ARQ operation. The upper layers 205 translate to protocol stacks formed for each individual service. For example, AMR codec/RTP/UDP/IP or FTP/TCP/IP can be the upper layer.
The MAC layer 215, connected to a plurality of ARQ entities 210, multiplexes at least one ARQ packet 220 and 230 to one HARQ packet 225, and performs HARQ operation on the HARQ packet 225. The physical layer 217 performs an operation of transmitting/receiving the HARQ packet 225 over a radio channel. The ARQ packets 220 and 230 and the HARQ packet 225 will be described separately. The ARQ packets 220 and 230 are the packets reconfigured so that outer ARQ can be performed on the data delivered from the upper layers 205. The HARQ packet 225 is the packet that is actually transmitted/received on a radio channel through an HARQ operation.
Structures of the ARQ packets 220 and 230 and the HARQ packet 225 will be described. The ARQ packets 220 and 230 each include a sequence number (SN) 231, Size information 233, Framing header (HDR) information 235, and a payload 237. If an IP packet is delivered from the upper layer 205 to the ARQ entity 210, the whole or a portion of the IP packet can be transmitted according to radio channel status or scheduling status. Reconfiguring the original packet delivered in an appropriate size from the upper layer 205 is called ‘framing’, and the framing HDR information 235 is the information with which a receiver can restore the framed packet to its original packet. In addition, the Size information 233 indicates a size of the ARQ packets 220 and 230, and the SN 231 is sequentially assigned to the ARQ packets 220 and 230. The ARQ entity 210 performs an outer ARQ using the SN. The HARQ packet 225 includes a Multiplexing header (HDR) 240, and a payload. The Multiplexing HDR 240 includes multiplexing information for at least one ARQ packet 220 or ARQ packet 230. For example, an identifier (ID) of the ARQ entity 210 can be the Multiplexing HDR 240.
FIG. 3 is a diagram illustrating an ARQ operation and an HARQ in a general LTE system. In the following description, it will be construed that the ARQ operation is equal to an outer ARQ operation.
As illustrated in FIG. 3, an HARQ entity is divided into a transmitter's HARQ entity 372 and a receiver's HARQ entity 312 according to transmission/reception operation. The transmitter's HARQ entity 372 takes charge of transmission/retransmission of an HARQ packet, and the receiver's HARQ entity 312 takes charge of soft-combining on an HARQ packet and acknowledgement/negative acknowledgement (ACK/NACK) signal transmission. Because a UE and an ENB each have a transmitter and a receiver, the transmitter and the receiver are not limited herein to any one of the UE and the ENB. The UE and the ENB may also correspond to a mobile station (MS) and a base station (BS), respectively.
Because services of various types should be provided through the HARQ, the transmitter and the receiver include their own upper layer entities (not shown), ARQ entities 380 and 305, and multiplexing/demultiplexing blocks 375 and 310, respectively. The multiplexing block 375 in the transmitter inserts the Multiplexing HDR 240 in the data generated by several upper layers, and delivers the resulting data to the transmitter's HARQ entity 372, and the demultiplexing block 310 in the receiver delivers the multiplexing information of data provided from the receiver's HARQ entity 312 to a proper upper layer.
A plurality of HARQ processors 355 to 370 and 315 to 330 included in the transmitter's HARQ entity 372 and the receiver's HARQ entity 312 are basic units for taking charge of transmission/reception of the HARQ packet 225. The transmitter's HARQ processors 355, 360, 365 and 370 take charge of transmission/retransmission of the HARQ packet 225, and the receiver's HARQ processors 315, 320, 325 and 330 take charge of reception and soft-combining of an HARQ packet, and ACK/NACK information transmission. The HARQ processors are provided in the transmitter and the receiver in pairs, and continuous transmission/reception is possible by providing a plurality of HARQ processors in one HARQ entity.
In the transmitter, each of the HARQ processors includes operations of transmitting a user packet, receiving ACK/NACK information in response thereto, and performing retransmission. Therefore, if there is only one HARQ processor, the HARQ processor cannot transmit other packets until it transmits user data and receives ACK/NACK information in response thereto. However, in the case where several HARQ processors are provided in the transmitter and the receiver, while one processor waits for the ACK/NACK information, the other processors can transmit data. Therefore, continuous transmission/reception is possible by providing a plurality of HARQ processors in each HARQ entity as shown in FIG. 3.
A description will now be made of basic operations of the HARQ processors 355 to 370 and 315 to 330.
The transmitter's HARQ processors 355, 360, 365 and 370 perform channel coding on the data received from the multiplexing block 375 and transmit the channel-coded data to the receiver. Also, the transmitter's HARQ processors 355, 360, 365 and 370 simultaneously store the channel-coded data in a buffer (not shown) to retransmit it later. Further, the transmitter's HARQ processors 355, 360, 365 and 370 flush the data stored in the buffer upon receipt of HARQ ACK information for the transmitted data from the receiver, and retransmit the data stored in the buffer upon receipt of NACK information for the transmitted data from the receiver.
The receiver's HARQ processors 315, 320, 325 and 330 perform channel decoding on the data received from the transmitter over a physical channel, and determine whether there is an error through a CRC operation. If there is an error, the receiver's HARQ processors 315, 320, 325 and 330 store the received data in a buffer, and transmit an HARQ NACK signal to the transmitter. If retransmitted data for the received data is received due to the error, the receiver's HARQ processors 315, 320, 325 and 330 soft-combine the data stored in the buffer with the retransmitted data, and then determine whether there is an error. If an error is determined to still exist, the receiver's HARQ processors 315, 320, 325 and 330 transmit an HARQ NACK signal to the transmitter, and repeat the above process. If it is determined that the error is solved, the receiver's HARQ processors 315, 320, 325 and 330 transmit an HARQ ACK signal to the transmitter, and deliver user data to the demultiplexing block 310.
The reception success rate can increase through the HARQ operation of retransmitting and soft-combining the defective HARQ packet. However, it is inefficient to achieve a very low Block Error Rate (BLER) only with the HARQ operation, for the following two reasons.
First, if there is an error in an HARQ ACK/NACK signal, the HARQ operation cannot detect the error.
Second, because HARQ transmission/retransmission is performed within a relatively short time, the HARQ operation cannot obtain a time diversity gain. As a simple example, if an MS falls in deep fading, it is hard to successfully transmit the HARQ packet through HARQ retransmission.
In order to overcome the limitation of the HARQ operation, there is a need to perform the outer ARQ operation.
The outer ARQ operation is performed in units of ARQ packets. The transmitter's ARQ entity 380 attaches SNs 231 to the ARQ packets 220 and 230 shown in FIG. 2 before transmission. The receiver's ARQ entity 305 verifies the SNs 231 of the received ARQ packets 220 and 230, and determines whether there is any missing ARQ packet. For example, if the receiver's ARQ entity 305 has received an ARQ packet with SN=X and an ARQ packet with SN=X+2, but has failed to receive an ARQ packet with SN=X+1, the receiver's ARQ entity 305 sends a retransmission request for the ARQ packet with SN=X+1 to the transmitter's ARQ entity 380.
FIG. 4 is a block diagram illustrating a detailed structure of the ARQ entities 305 and 380 for performing an outer ARQ operation in a general LTE system. Herein, the outer ARQ operation is performed by the transmitter's ARQ entity 380 and the receiver's ARQ entity 305.
The transmitter's ARQ entity 380 includes a transmission buffer 405, a header inserter 410, an ARQ control block 415, and a retransmission buffer 420. The transmission buffer 405 stores the packets delivered from an upper layer. The transmission buffer 405 also delivers as much of the upper layer packet as the amount of data to be transmitted in the next transmission period, to the header inserter 410. If the amount of data to be transmitted in the next transmission period is not equal to a size of the upper layer packet, the transmission buffer 405 may only deliver a part of the upper layer packet by segmenting the upper layer packet, or may deliver a plurality of upper layer packets.
The header inserter 410 inserts SN information 231, Size information 233 and Framing HDR information 235 illustrated in FIG. 2 in the upper layer packet provided from the transmission buffer 405, thereby generating ARQ packets 220 and 230. The ARQ packets 220 and 230 are delivered to the retransmission buffer 420 and a lower layer (MAC/HARQ/PHY layer) 425. The retransmission buffer 420 stores ARQ packets for which the transmitter has failed to receive an ACK signal from the receiver. The ARQ control block 415 discards a corresponding ARQ packet upon receipt of an ARQ ACK signal from the receiver, and schedules retransmission of the corresponding ARQ packet upon receipt of an ARQ NACK signal. The lower layer 425 includes a MAC layer, an HARQ entity, and a PHY layer. The lower layer 425 also multiplexes ARQ packets to an HARQ packet and then transmits the HARQ packet to the receiver over a physical channel.
The receiver's ARQ entity 305 includes a reassembly block 445, a reception buffer 435, and a retransmission management block 440. A lower layer (MAC/HARQ/PHY layer) 430 receives an HARQ packet transmitted from the transmitter over a physical channel, and delivers demultiplexed ARQ packets to the receiver's ARQ entity 305.
The reception buffer 435 stores the ARQ packet received from the lower layer 430 according to its SN, and delivers ARQ packets capable of being reassembled to the reassembly block 445. The retransmission management block 440 verifies SNs of the ARQ packets stored in the reception buffer 435. The retransmission management block 440 also delivers ARQ ACK signals for the received ARQ packets and ARQ NACK signals for the missing ARQ packets to the transmitter's ARQ entity 380. The reassembly block 445 reassembles the original upper layer packet using the ARQ packets depending on the framing headers of the ARQ packets delivered from the reception buffer 435, and then delivers the upper layer packet to an upper layer.
As described above, the HARQ and outer ARQ operations retransmit the missing packets, and they are fundamentally equal to each other. However, in the mobile communication system supporting the outer ARQ, there is no solution to errors that may occur when the transmitter wrongly receives an ARQ ACK/NACK signal, or a feedback signal, from the receiver's ARQ entity 305. Therefore, the mobile communication system, similar to the LTE system using both the HARQ and ARQ, requires a technology to determine whether there is an error in an ARQ ACK/NACK signal.
Accordingly, there is a need for an improved system and method for determining whether there is an error in an ARQ ACK/NACK signal.